Navigation data, for the purpose of this invention, is comprised of (but not limited to) a body's position, velocity and orientation. Navigation is conducted by means of devices depending on external sources, such as GPS, and by means of independent devices such as inertial measurement unit (IMU). Accurate navigation requires physically big IMU sensors having relatively large physical dimensions, typically larger than 10 cubical centimeters as well as complex mounting techniques. In many situations, it is useful for personnel traveling on foot to carry a portable, light-weight and accurate navigation system. Such situations may include the training of military and rescue forces, and military and rescue operations. Such a navigation system may include an inertial measurement unit (IMU) for determining acceleration and orientation of the measured body. When acceleration and orientation are known, calculation of a current position may be made on the basis of a previous position. An IMU will generally include accelerometers oriented along various axes for measuring accelerations along those axes, and gyroscopes oriented along various axes for detecting changes of orientation with respect to those or other axes. Often, the axes selected are mutually orthogonal X, Y, and Z axes. Orientations are often defined in terms of roll, pitch and yaw.
However, position calculation on the basis of a portable IMU is subject to systematic and random errors. Various correction methods have been developed in order to compensate for these errors. Some of the correction methods rely on the sensor Error Model, which is a set of mathematical equations which describe the behavior of the sensor outputs in terms of random variables and their associated probability distributions. Solutions that have been proposed or adopted for compensating for errors include: zero velocity update sensors that identify points of a gait or motion where velocity may be assumed to be zero, an assumption of no roll to eliminate spurious results, periodic comparison with Global Positioning System (GPS) data, and measurements of the earth's magnetic field to verify orientation. Although magnetic field augmentation is widely used to correct orientation data, the direction of the local magnetic field is vulnerable to external interference, reducing the accuracy of yaw measurements.
It should be noted that the description of embodiments of the present invention herein below exemplifies aspects of the invention mostly by discussing implementations using sensors of the micro-electromechanical system (MEMS) sensors type. It would be apparent however for a person skilled in the art that other types of small-sized, small-weight and/or small-energy-consumption may be used instead of MEMS sensors. Micro inertial sensors, such as MEMS inertial sensors, due to their small size, light weight, and low power consumption, are especially attractive for use in navigation systems in all applications sensitive to power, space and weight limitations. However, to a much greater extent than high-end, larger mechanical or optical inertial sensors, MEMS sensors are usually characterized by lack of stability, and poor observability (the ability to infer internal states from observed quantities). In addition, error models for MEMS sensors are not as well developed as for other inertial sensors, making the development of accurate correction algorithms for a MEMS-based inertial system difficult.
The sensor errors of a MEMS IMU may be modeled as random processes. A separate and specific model must be constructed and verified for each type of sensor, and, in fact, for each specific manufactured sensor of each type. The model may then be utilized in a Kalman Filter, or other type of estimator, in which the model is used to estimate the inertial sensor errors. However, the lack of stability of MEMS IMU sensor results may frustrate this effort.
There is therefore a need for accurate MEMS-based inertial navigation system.
It is an object of the present invention to provide an accurate MEMS-based inertial navigation system. Such a system may be used also as a portable navigation device.